Difluorostyrene derivatives and corresponding liquid-crystalline media, such as, for example, containing compounds of the formula ##STR4## have already been disclosed in EP 0 325 796 A1. However, these compounds tend to be chemically unstable due to illumination of HF.
Various compounds having liquid-crystalline properties and containing a terminal CF.sub.3 group have already been disclosed (U.S. Pat. No. 4,330,426; U.S. Pat. No. 4,684,476; J. C. Liang and S. Kumar, Mol. Cryst. Liq. Cryst. 1987; Vol. 142, pp. 77-84). However, these compounds frequently have a highly smectogenic character and are less suitable for many practical applications.
However, in view of the very wide variety of areas of application of compounds of this type having a high .DELTA..epsilon., it was desirable to have available further compounds which have properties precisely customized for the particular applications.